1. Field of the Invention
The invention relates to furnaces implementing the Czochralski (“CZ”) crystal growth process and methods operating same.
2. Description of the Prior Art
The Czochralski crystal growth technique is a commonly used method for industrial growth of rare earth oxyorthosilicates materials for photonic and electronic applications, including by way of nonlimiting example molecular imaging, homeland security and well logging industry applications. However, oxyorthosilicates while grown in the form of large single crystals become very susceptible to cracking, as may also display substantial nonuniformities in their optical and scintillation properties.
Cracking is a result of nonuniform thermal stress distribution in the volume of the crystal. The amount of thermal stress accumulated in the material depends strongly on values of thermal expansion coefficients usually measured along the main crystallographic axes. The differences between these coefficients in oxyorthosilicates may be very large. As a result, a substantial amount of stress is generated in the volume of the crystal during crystal growth and subsequent cooling processes. These stresses are amplified by differences in thermal gradients in a Czochralski furnace in both radial and axial directions. When thermal stresses exceed certain threshold values, the crystal releases accumulated internal stress energy and shatters.
Nonuniformities of crystal scintillation and optical properties are associated mostly with differences in the distributions of codopants and impurities in the volume of the material. Since most of the codopants and impurities accumulate on the surface of the melt during the growth process, any instability in the convection flow of the melt and disturbance of Marangoni flow will have a direct influence on the crystal composition.
Therefore, in order to minimize both the cracking effect and nonuniformity issues it is necessary to maintain continuous control over the thermal gradients in the crystal growth environment during the entire growth process.
Oxyorthosilicate crystals are usually grown in the form of long cylindrical boules, therefore temperature differences between the top and the bottom of the furnace should be maintained relatively low to avoid crystal cracking. However, relatively high thermal gradients between the crystal seed and the melt are necessary to inmate the growth process. Therefore, to achieve the optimal thermal environment in a Czochralski growth system, a common practice is to use afterheaters to control furnace axial gradients in the later stages of growth and cooling down processes. This solution requires a complicated and expensive double-coil RF crucible heating system, especially when used with high temperature growth processes. Moreover, in the traditional Czochralski furnace it is very difficult to control the behavior of the melt in the later stages of the growth process when convection flow of the melt and Marangoni flow are disturbed by a drop of the melt level in the crucible. It causes instabilities within the crystal-melt interface that affect the distribution of codopants in the volume of the crystal.
Thus, a need exists in the art for a Czochralski (“CZ”) furnace that initially achieves high vertical thermal gradients in the crucible during the initial crystal formation process, so as to encourage crystal boule formation.
Another need exists in the art for a Czochralski (“CZ”) furnace that after initial crystal formation alters its heating properties to enhance uniformity of radial thermal gradients and lower vertical thermal gradients in the crucible crystal/melt line interface, so as to reduce likelihood of crystal boule cracking as the boule grows.
Yet another need exists in the art for a Czochralski (“CZ”) furnace that minimizes instabilities in the crystal codopant distribution within the crystal boule (that would otherwise negatively impact crystal scintillation and optical property uniformity) potentially caused by convection flow and Marangoni flow disturbance as the remaining melt volume in the crucible diminishes, without the need for additional superheating hardware in the RF heater.